feat: optional centralized storage for sidecar metadata and previews (#1045)

Add an opt-in 'centralized' sidecar storage mode alongside the default
'alongside' layout. In centralized mode, .metadata.json sidecars and
preview assets live under a configurable root (sidecar_storage_path,
default <settings_dir>/sidecars), mirroring the library-relative
directory structure: <root>/<library>/<root_basename>/<rel_dir>/.

Backend:
- settings: sidecar_storage_mode / sidecar_storage_path with validation;
  changing either refreshes the preview allowlist
- config: centralized root added to preview-serving allowlist
- lifecycle: delete / move / rename / folder-rename / folder-delete and
  undoable-delete staging all operate on the mirror tree in centralized
  mode (model files themselves never move); EXDEV-tolerant cross-
  filesystem moves
- scanners: pending-hash filesystem scan walks the mirror tree in
  centralized mode; preview discovery reads from the sidecar dir;
  .civitai.info stays co-located in both modes
- migration: SidecarMigrationUseCase moves sidecars+previews between
  layouts both directions (keep-newer conflict resolution, preview_url
  rewriting, WebSocket progress), exposed as POST+GET
  /api/lm/sidecars/migrate with a mode guard (force=true for the
  settings-first flow)

Frontend:
- settings modal: sidecar storage section (mode select + path input with
  browse/validation), mode-change confirmation offering immediate
  migration (force=true), and a 'Migrate Sidecars Now' action
- i18n keys synced to all locales ([TODO: Translate] placeholders)

Docs: metadata-json-schema.md gains a storage-location section;
AGENTS.md records the sidecar_paths helper convention.

py/utils/metadata_manager.py

@@ -8,7 +8,7 @@ from typing import Any, Dict, Optional, Type, Union, cast
 from .models import BaseModelMetadata, CheckpointMetadata, EmbeddingMetadata, LoraMetadata
 from .file_utils import normalize_path, find_preview_file, calculate_sha256, calculate_autov3
 from .lora_metadata import extract_lora_metadata, extract_checkpoint_metadata
-from .sidecar_paths import get_metadata_path, resolve_metadata_path
+from .sidecar_paths import get_metadata_path, get_preview_dir, resolve_metadata_path
 
 logger = logging.getLogger(__name__)
 
@@ -189,6 +189,10 @@ class MetadataManager:
         temp_path = f"{metadata_path}.tmp"
         
         try:
+            # Centralized sidecar mirrors may not exist yet (unlike the model's
+            # own directory in alongside mode, which always does).
+            os.makedirs(os.path.dirname(metadata_path), exist_ok=True)
+
             # Convert to dict if needed
             if isinstance(metadata, BaseModelMetadata):
                 metadata_dict = metadata.to_dict()
@@ -251,10 +255,9 @@ class MetadataManager:
             
         try:
             base_name = os.path.splitext(os.path.basename(file_path))[0]
-            dir_path = os.path.dirname(file_path)
-            
+
             # Find preview image
-            preview_url = find_preview_file(base_name, dir_path)
+            preview_url = find_preview_file(base_name, get_preview_dir(file_path))
             
             # Calculate file hash
             start_hash_time = time.perf_counter()
@@ -378,15 +381,16 @@ class MetadataManager:
         # Check if preview exists at the current location
         preview_url = metadata.preview_url
         if preview_url:
-            # Get directory parts of both paths
-            file_dir = os.path.dirname(file_path)
+            # Get directory parts of both paths; the preview directory is the
+            # sidecar/preview dir (the model's own dir in alongside mode, the
+            # centralized mirror otherwise).
+            file_dir = get_preview_dir(file_path)
             preview_dir = os.path.dirname(preview_url)
             
             # Update preview if it doesn't exist OR if model and preview are in different directories
             if not os.path.exists(preview_url) or file_dir != preview_dir:
                 base_name = os.path.splitext(os.path.basename(file_path))[0]
-                dir_path = os.path.dirname(file_path)
-                new_preview_url = find_preview_file(base_name, dir_path)
+                new_preview_url = find_preview_file(base_name, file_dir)
                 if new_preview_url:
                     metadata.preview_url = normalize_path(new_preview_url)
                     need_update = True